Track circuits may be used in the railroad industry to detect the presence of a train in a block of track. Track circuit hardware may include transmitters and receivers configured to work with coded AC, coded DC, or audio frequency (AF) signals. Different track circuits may function in different ways to detect trains and may therefore have different hardware requirements. For example, some track circuits (such as AC overlay circuits) may have a transmitter configured to transmit a signal through the track rails at one end of a block of track and a receiver connected to the rails at the other end of the block and configured to detect the signal. Other than the connection through the track rails, there may typically be no connection between the transmitter and receiver for a block. When a train is present in a block of track monitored by a track circuit, the train may shunt, or short, the two rails, with the result that no signal is received at the receiver. Thus, the receiver may use the presence or absence of a detected signal to indicate whether or not a train is present in the block. In some other track circuits, sometimes referred to as constant warning time circuits, a transmitter may transmit a signal over a circuit formed by the rails of the track and one or more shunts positioned at desired approach distances from the transmitter. A receiver may detect one or more resulting signal characteristics, and a logic circuit such as a microprocessor or hardwired logic may detect the presence of a train and may determine its speed and distance from a location of interest such as a crossing. The track circuit may detect a train and determine its distance and speed by measuring impedance changes due to the train's wheels and axle acting as a shunt across the rails and thereby effectively shortening the length (and hence the impedance) of the rails in the circuit. Those of skill in the art will recognize that other configurations of track circuits are possible.
Transmitters used in some track circuits may comprise H-bridge circuits. These H-bridges may comprise four transistors, such as MOSFETs, used as switches to perform signal generation for transmitting only. By opening and closing the switches, the H-bridge can be controlled to direct a signal from a power supply through an attached track load (i.e. the railroad track) with a defined voltage, current, and polarity. A transmitter using a four-transistor H-bridge can transmit a signal to the track load, and a separate receiver can receive the signal to detect the presence or absence of a train on the track. Transmitters using four-transistor H-bridges may need different physical hardware configurations for different types of track circuits, and may be unable to function as transceivers for the track circuit, so a separate receiver may be required.